Welcome to the forum. Just do a Google search for irish pub t-shirts and you will come up with hundreds of places that sell them. You can even get personalized ones made up. Happy T-shirt hunting!
One of my favorite shirts I brought back from Ireland was from a pub in Dublin called Foley's. It's one of the pubs I frequented at least thrice a week, and a guy I was seeing over there bought me the shirt as a gift. So comfortable and pretty cute!